The present invention relates to a dry-etching method whereby interconnect patterning is made through the dry-etching of a Cu-containing aluminum film formed on a substrate, and more particularly, to a Cu-containing aluminum film dry-etching method that prevents the generation of copper residues during etching.
As the design rule of semiconductor devices becomes increasingly finer these years, MOS field-effect transistors have occupied the major portion of the elements constituting semiconductor devices. Also as the design rule of MOS field-effect transistors becomes finer, the multi-layered interconnect structure itself becomes increasingly finer as well. For example, the damascene method has come to be employed in the copper interconnect formation process for the fabrication of semiconductor devices to which design rules of 0.13 μm or narrower are applied. On the other hand, the dry etching method has been employed in the interconnect formation process for semiconductor devices to which the 0.15 μm design rule is applied because of its process simplicity. In this case, the major interconnect material is aluminum. Downsizing of the multi-layered interconnect structure, however, is approaching its limits; indeed, the minimum pitches between neighboring lines are currently set at 0.40 μm or shorter.
As one of the excellent techniques for the formation of high-quality aluminum interconnects, a fine pattern interconnect forming method is generally known that uses the plasma dry-etching system for patterning a Cu-containing aluminum film. This dry-etching technique has been used in a wide range of semiconductor applications. This dry-etching method employs mixtures of chlorine gas, boron trichloride gas and nitrogen gas in most cases.
Then a conventional dry-etching method, specifically, an interconnect pattern formation method employing the dry-etching of the Cu-containing aluminum film will be described below with reference to attached drawings.
FIGS. 7A-7B are sectional views illustrating the steps of the conventional dry-etching method.
Referring now to FIG. 7A, a first barrier metal film 3 of a thickness of 20 nm, a Cu-containing aluminum film 4 of a thickness of 300 nm and a second barrier metal film 5 of a thickness of 20 nm are formed one after another on a silicon substrate 1 with a dielectric film 2(silicon oxide film) interposed between the substrate 1 and the film 3. The first barrier metal film 3 and the second barrier metal film 5 have the respective multi-layered structures composed of TiN and Ti layers. The mass ratio between aluminum and copper in the Cu-containing aluminum film 4 is 99.5%:0.5%. Hereafter, the multi-layer structure composed of the first barrier metal film 3, Cu-containing aluminum film 4 and the second barrier metal film 5 is referred to as a TiN/Ti/AlCu/TiN/Ti film 6. Each layer in the TiN/Ti/AlCu/TiN/Ti film 6 is deposited sequentially by sputtering, for example.
Next a resist film is formed on the TiN/Ti/AlCu/TiN/Ti film 6, and this resist film is exposed and developed for patterning. Then, as shown in FIG. 7A, a resist pattern 7 is formed that covers the interconnect region.
Subsequently, the TiN/Ti/AlCu/TiN/Ti film 6 is dry-etched, with the resist pattern 7 being used as a mask, by the use of an etching gas of which major content is chlorine gas, in order to for an interconnect 6A consisting of the TiN/Ti/AlCu/TiN/Ti film 6, as shown in FIG. 7B.
As the chamber pressure in the dry etching apparatus is reduced, the ratio of gas radicals (radicals generated from the plasma-activated etching gas) falling vertically on the silicon substrate 1 increases in general. As a result, it becomes easy to for an interconnect 6A of a cross-section having clear-cut vertical straight sides, and then the preferable interconnect shape can be provided. Namely, the interconnect pattern can be formed with high accuracy in shape. On the other hand, if the chamber pressure is reduced, copper residues 8 are likely to appear around the interconnect 6A, in particular, between interconnects, as shown in FIG. 7B. If the copper residue 8 is created, it causes leak current between interconnects, for example, and eventually leads to lower manufacturing yields in the semiconductor device manufacturing process.
To prevent the generation of copper residue during the dry-etching of the Cu-containing aluminum film, an aluminum ring (hereafter, Al ring) is occasionally mounted on the rim of the lower electrode on which the substrate is mounted for treatment in the plasma dry-etching apparatus.
FIG. 8A illustrates a schematic view of a structure of a conventional plasma dry etching apparatus, specifically, inductively coupled plasma (ICP) apparatus, while FIG. 8B is a top view of the lower electrode of the apparatus shown in FIG. 8A on which the substrate is mounted.
As shown in FIGS. 8A and 8B, the lower electrode 12 on which a wafer 11, the substrate to be treated is mounted is installed on a holder 13, at the bottom of the chamber 10 where plasma is generated. The inner wall of the chamber 10 is coated with anodized aluminum. The lower electrode 12 is connected to the bias power source 14 located outside the chamber 10. An Al ring 15 is mounted on the rim of the lower electrode 12 so as to surround the wafer 11. An inductive coupler coil 16 is installed on the top of the chamber 10, while this inductive coupler coil 16 is connected to a high frequency power source 17 located outside the chamber 10.
By employing such an Al ring in the plasma etching apparatus, namely, employing such a dry etching apparatus shown in FIGS. 8A and 8B, it becomes possible to prevent the generation of copper residue during the dry etching process for the Cu-containing aluminum film. The use of an Al ring, however, may cause a difference in the etching rates of the Cu-containing aluminum film between the wafer peripheral and the wafer center. Further, as the properties of the Al ring change with time, the etching rate as well comes to change with time. As a result, the dry etching process becomes difficult to control, and eventually the manufacturing yield of the semiconductor device manufacturing process falls.
In addition to the Al ring, there are other conventional dry-etching methods for etching the Cu-containing aluminum film preventing the generation of copper residue. For example, the temperature of the lower electrode of the plasma dry etching apparatus is raised; and an excessive over-etching is employed.
First, the former method for raising the temperature of the lower electrode will be explained for the case where this method is applied to the conventional dry etching process shown in FIGS. 7A and 7B. A case of using the ICP etching apparatus is exemplified that is operated under the etching conditions described in Table 1.
TABLE 1BCl3/Cl2/N290/90/100(mL/min)Pressure0.4(Pa)ICP/RF300/200(W)Temperature of lower80(° C.)electrodeOver etching rate30%
As described in Table 1, the etching gas is a mixture of boron trichloride (BCl3), chlorine gas (Cl2) and nitrogen gas(N2). The gas flow rates for BCl3, Cl2 and N2 are 90 mL/min, 90 mL/min and 100 mL/min, respectively, all under the standard state. The chamber pressure is 0.4 Pa; the high frequency power (ICP) applied to the inductive coupler coil is 300W; and the high frequency power (RF) applied to the lower electrode is 200W. The temperature of the lower electrode is set at 80° C., higher than the usual temperature, 50° C. The over etching rate is set at 30%.
In the present specification, the over etching rate is the ratio to the main etching of the over etching that is performed after the main etching so as to ensure the etching of the film. The main etching is defined by (film thickness before etching)/(etching rate).
If the TiN/Ti/AlCu/TiN/Ti film 6 is dry-etched with the resist pattern 7 used as a mask under the etching conditions shown in Table 1 (see FIGS. 7A and 7B), the temperature of the TiN/Ti/AlCu/TiN/Ti film 6 as well tends to become high since the temperature of the lower electrode is high. As a result, the copper residue 8 is not likely to be produced because the TiN/Ti/AlCu/TiN/Ti film 6 (particularly Cu-containing aluminum film 4) reacts well with gas radicals. However, as shown in FIG. 9, the interconnect 6A tends to present an inversely tapered shape rather than a cross-section having clear-cut vertical straight sides because of the side etching effect.
Next explained is the excessive over-etching method, which is applied to the conventional dry-etching process shown in FIGS. 7A and 7B under the etching conditions shown in Table 2 for the ICP etching apparatus.
TABLE 2BCl3/Cl2/N290/90/100(mL/min)Pressure0.4(Pa)ICP/RF300/200(W)Temperature of lower50(° C.)electrodeOver etching rate70%
The differences from Table 1 with regard to etching conditions are that the temperature of the lower electrode is 50° C. in Table 2, lower than that in Table 1, and that the over-etching rate is 70% in Table 2, higher than that in Table 1.
If the TiN/Ti/AlCu/TiN/Ti film 6 is dry-etched, with the resist pattern 7 being used as a mask, under the etching conditions shown in Table 2 (see FIGS. 7A and 7B), the copper residue 8 is removed by the over-etching, as a result of the lift-off of the dielectric film 2 serving as the base layer of the TiN/Ti/AlCu/TiN/Ti film 6. Meanwhile, the resist pattern 7 serving as an etching mask is likely to be lost during over-etching and thus the shape of the top of the interconnect 6A tends to collapse, referring to FIG. 10. Namely, it becomes difficult to for an interconnect 6A of the precise target shape.
As described so far, if one tries to for interconnect of a precise shape by reducing the chamber pressure and patterning the Cu-containing aluminum film based on the conventional dry-etching method, copper residue is likely to be formed and thus leak current may run between interconnects.
Meanwhile, if one tries to for interconnect by raising the temperature of the lower electrode or conducting the excessive over-etching based on the conventional dry etching method, in order to pattern the Cu-containing aluminum film and prevent the generation of copper residue, the cross-section of the interconnect tends to present an inversely tapered shape or the shape of the top of the interconnect is likely to collapse. Such degradation in the shape of interconnect leads to an interconnect resistance larger than expected levels and then results in significantly lowered performance in the manufactured multi-layer interconnect.
In other words, the precise shaping of interconnect and the prevention of copper residue are in the relation of trade-off, and they are not attained at the same time by the conventional dry etching method. As a result, problems are often posed in the multi-layered interconnect structure.